Electrical power is mainly and preferably supplied via balanced three-phase systems. Unequal loads between the individual phases result in such three-phase systems becoming unbalanced, which furthermore results in power losses or damage in electrical machines which are connected to the three-phase system. Unbalanced three-phase systems are in this case distinguished not only by unbalanced currents and phases but, in particular, also by unbalanced voltages, to be precise owing to the different voltage drops in the individual paths.
The said unbalanced voltages in particular have a negative effect on electrical machines which are connected to the three-phase system. Depending on the extent of the unbalance, increased power losses occur first of all, then reduced life, and, finally, machine failure. For this reason, International Standards and Recommendations have been issued in which the maximum permissible level of unbalance in a three-phase system has been defined (Engineering Recommendation, IEC--The Electrical Council, London, June 1975, page 16).
An unbalance in a three-phase system may occur, for example, due to a single-phase induction furnace, due to a single-phase propulsion system which is connected directly to the three-phase system, or due to an arc furnace, which in principle admittedly represents a three-phase load, but in which an unbalance can occur briefly when one or two arcs are quenched.
One known method to compensate for unbalanced loads was proposed by Ch. P. Steinmetz (L. Gyugyi et. al, Principle and Applications of static, thyristor-controlled Shunt Compensators, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-97, No. 5, September/October 1978, pages 1935to 1945 and, in this case, in particular page 1936, left-hand column). Steinmetz was able to show that a resistive load contained between two phases of a three-phase system can be compensated for by connecting a capacitance of suitable size and an inductance of suitable size between the phase to which the resistive load is connected and the phase which is not loaded by the resistive load. In this case, Steinmetz assumed that this was a pure resistive load. If this condition is not satisfied, then additional complications are involved in order to make it possible to correct the power factor as well.
The aim of a compensating load may theoretically be achieved firstly by eliminating the so-called negative-sequence system (L. Gyugyi et. al, Principle and Applications of static, thyristor-controlled Shunt Compensators, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-97, No. 5, September/October 1978, page 1937, right-hand column), and secondly by correcting the power factor. This can be achieved in a known manner by providing a compensation circuit in triangular form, comprising reactive elements such as capacitors and inductances.
With the availability of power switches--such as thyristors--the power factor can be corrected as required, that is to say it has even been possible to balance three-phase systems with widely varying loads. A circuit developed on this principle is described and explained in the abovementioned article by Gyugyi et. al. (page 1942, right-hand column, FIG. 14).
This known circuit for compensating for unbalanced loads has the disadvantage, however, that no volt-ampere optimization is provided. For this reason, the losses are correspondingly high.